Semiconductor instruments have been mainly used for transmission parts of radio communication equipment such as mobile phones using radio-frequency waves and for amplifying radio-frequency signals to defined power necessary for conducting communication with wireless base stations.
As to mobile phones, longtime continuous conversation has been demanded in general. In order to realize longtime continuous conversation, a battery needs to have a power supply with a large capacity and power consumption needs to be low in an internal circuit of a mobile phone during a telephone conversation.
An electromagnetic wave is continuously transmitted to a base station during a conversation of a mobile phone. The transmission power is different depending on a communication method and a use condition of the mobile phone and increases up to about 4W. A component called a power amplifier in the mobile phone amplifies a signal to such a power. The power amplifier is one of the components having the largest power consumption in an internal circuit of the mobile phone. Therefore, it is important to suppress power consumption of the power amplifier, that is, to improve power added efficiency of the power amplifier in order to realize a longtime continuous conversation.
The following will discuss a method of improving power added efficiency of a conventional amplifier in accordance with the accompanied drawings.
The power amplifier is generally constituted by an FET or a bipolar transistor that serves as an amplifying element, a bias circuit for supplying current to the amplifying element, a matching circuit, and a substrate for mounting the transistor and the circuits thereon. In this case, a MOS transistor, an FET made of GaAs, or a heterojunction bipolar transistor made of GaAs is often used as the amplifying element. Further, the power amplifier is generally configured as a multistage amplifier circuit using a plurality of amplifying elements to obtain a desired gain. According to “Basics and Application of MMIC Technology” (written by Yasuyuki Ito and Nao Takaki, published by REALIZE INC.), regarding an output matching circuit connected to the output side of a final amplifying element in a power amplifier, power added efficiency can be improved by using the configuration of FIG. 6.
In FIG. 6, on the output side of a final-stage amplifying element Tr, a first matching circuit U1, a resonant circuit U2, and a second matching circuit U3 are placed in series between the final-stage amplifying element Tr and an antenna terminal OUT. Besides, reference numeral 3 denotes a current supply terminal for the final-stage amplifying element Tr, and reference numeral 4 denotes a radio-frequency signal output terminal of the final-stage amplifying element Tr, An input matching circuit and so on are connected to the base of the final-stage amplifying element Tr.
The first matching circuit U1 is constituted by a first inductor 20, a first capacitor 21, and a first transmission line 22.
The resonant circuit U2 is constituted by a second inductor 23 and a second capacitor 24.
The second matching circuit U3 is constituted by a second transmission line 25, an open stub circuit 2-6, and a third capacitor 27.
In addition to frequency components including a communication signal (hereinafter, referred to as fundamental harmonics), radio-frequency frequency components inputted to the radio-frequency signal output terminal 4 from the final-stage amplifying element Tr include, frequency components twice as large as fundamental harmonics (hereinafter, referred to as 2nd harmonics), frequency components four times as large as fundamental harmonics, and frequency components six times as large as fundamental harmonics, and so on.
Frequency components other than the fundamental harmonics appear in this manner. This is because the final-stage amplifying element Tr is nonlinear and distortion occurs when the amplifying element amplifies a signal. Since frequency components other than the fundamental harmonics are unnecessary, if such frequency components can be suppressed, power consumed by frequency components other than the fundamental harmonics is reduced, thereby increasing power added efficiency of the power amplifier.
It is actually difficult to suppress all the frequency components other than the fundamental harmonics. However, since 2nd harmonics are the largest among the frequency components other than the fundamental harmonics, even when a matching circuit is designed only in consideration of the 2nd harmonics, a considerable effect can be obtained for improving power added efficiency.
The output matching circuit of FIG. 6 is designed in view of the 2nd harmonics. Power added efficiency is improved by the, principle below.
As to the resonant circuit U2 in the previous stage of the second matching circuit U3, values are set to produce resonance to 2nd harmonics, and the second matching circuit U3 does not affect 2nd harmonics. Therefore, matching of 2nd harmonics is determined by the first matching circuit U1, and these values are adjusted so as to have the maximum power added efficiency. Moreover, a matching condition for obtaining the maximum power added efficiency is available for the fundamental harmonics as well, and the second matching circuit U3 is adjusted so as to meet the condition.
In this way, matching is made to the fundamental harmonics and 2nd harmonics, so that power added efficiency is improved as compared with matching only to the fundamental harmonics.